A current shunt provides for indirect measurement of current values by measurement of the voltage developed across the current shunt by the current passing through the current shunt. Typical applications for current shunts include electricity usage control, over-current protection and metering of electricity consumption and generation. In use a current shunt of known resistance is provided in series with a load and the voltage developed across the current shunt by the load drawn current is measured. The current passing through the current shunt is then determined on the basis of Ohm's Law in view of the measured voltage and the known resistance of the shunt.
Certain applications, such as metering of electricity consumption and generation, require measurement to high accuracy over extended periods of time. For example in North America the ANSI C12.20 standard specifies an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere, such as IEC 62053, specify similar accuracy requirements. It can therefore be appreciated that the resistance of the current shunt must be known to high precision to enable a meter to meet regulated accuracy requirements. Although the shunt resistance is normally low to minimise power dissipation and undesirable circuit effects, the current shunt is nevertheless liable to heating with temperature drift giving rise to a change in resistance which may cause a loss of measurement accuracy in a shunt of ordinary temperature coefficient of resistance. Shunt resistors formed from manganin alloy are therefore widely used in view of their very low temperature coefficient of resistance. It may also be apparent that accurate current measurement depends on measurement of the voltage developed across the shunt being accurate and stable with temperature and lifetime. This is because a change in the transfer gain of the voltage measurement circuit or lack of precision in references used in the voltage measurement circuit will cause an error. It is normal for these reasons to perform a one-off factory calibration when the current shunt and the readout electronics are combined so that a factor related to the actual combined transfer function for current to measurement value, which is determined largely by the shunt resistor and voltage measurement, can be stored and used in subsequent measurements to achieve the desired precision.
An alternative known approach to measuring high values of current involves the use of a current transformer wound on a core, which is disposed around a conductor carrying current to be measured. The current transformer has the advantages over the shunt resistor of being less invasive and providing for isolation from the current carrying conductor. The current transformer is capable of measuring AC current only. The current transformer generates a current in the secondary coil, which is a ratio of the current in the primary conductor, and the secondary coil current is then turned into a voltage by a load, known as a burden resistor. Accurate measurement of the voltage across the burden resistor and accurate knowledge of the transfer function of the primary current to voltage across the burden resistor (i.e. combining the effect of number of turns, the magnetics and the burden resistor) are needed to measure the current accurately and precisely. As with the current shunt, one-off factory calibration is often performed to compensate for inaccuracies in some or all of the elements that contribute to the overall transfer function of primary current to measurement value.
Another approach uses a Hall current probe which is capable of measuring both AC and DC. In an open loop configuration the Hall current probe is, however, liable to non-linearity and temperature drift. In a closed loop configuration the Hall current probe provides an improvement with regards to non-linearity and temperature drift although the weight and size of the configuration increases significantly where higher currents are measured. It is further known to use the Rogowski coil current probe to measure high levels of current. Most known approaches to current measurement, such as by way of the shunt resistor, the current transformer, the Rogowski coil and the Hall current probe, are described and discussed in Current Sensing Techniques: A Review, Silvio Ziegler Robert C. Woodward and Herbert Ho-Ching lu, IEEE Sensors Journal, Vol. 9, No. 4, April 2009. The different known approaches have their respective advantages and disadvantages.
Load current measurement is often made in conjunction with line voltage measurement, which involves measuring the voltage between the conductors over which the current is delivered, in order to determine the electrical power. Often a resistive potential divider between the conductors is employed for line voltage measurement. High accuracy power calculation requires accurate and stable relative phase and frequency response of load current and line voltage measurements in order to accurately determine metrics such as the like of power factor, harmonic content and differences between active and reactive power amongst other things.
WO 2013/038176 describes an improved approach to the measurement of current. According to the approach of WO 2013/038176 a current sensor, such as a current shunt, a current transformer, a Hall current probe or a Rogowski coil, is disposed as described above relative to a conductor to sense a load drawn current flowing through the conductor. A reference signal which is known to high precision is applied to the current sensor whereby the current sensor is responsive to both the load drawn current signal and the applied reference signal. The output signal from the current sensor is acquired and the part of the output signal corresponding to the reference signal is extracted from the output signal. Then the transfer function of the current sensor and the current sensor processing chain is determined on the basis of the reference signal and the extracted part of the output signal corresponding to the reference signal. Thereafter the actual load drawn current flowing through the conductor is determined in dependence on the transfer function and the load drawn current as sensed by the current sensor. Accuracy of measurement of the load drawn current therefore depends on the reference signal being known to high precision instead of the current sensor and its processing chain being known to high precision as according to the previously described approaches. The lack of reliance on the known precision of the current sensor means a lower quality sensor may be used. There is also less need for initial calibration and periodic subsequent recalibration of the current sensor and its processing chain. Furthermore the approach of WO 2013/038176 addresses drift of the current sensor and its processing chain arising from the like of ageing and temperature change and also provides for additional functionality, such as the detection of tampering with electricity consumption meters.
The approach of WO 2013/038176 relies on the precision and stability of the reference signal. WO 2013/038176 describes a current reference circuit which is operative to set the reference signal applied to a current sensor by a signal source. The current reference circuit comprises a voltage controlled current source comprising a current mirror which is driven by a bias voltage provided by an amplifier which is in turn driven by an output from a silicon bandgap reference. WO 2013/038176 further describes different approaches to calibration of the current reference circuit.
Normally the reference signal of the apparatus of WO 2013/038176 is of much smaller amplitude than the load drawn current signal. Even so, the present inventors have appreciated that increasing the amplitude of the reference signal increases the signal to noise ratio and thereby provides for accuracy of current measurement. On the other hand, an increase in the amplitude of the reference signal increases the power consumption of the current measurement apparatus. Although the power supply for the current reference circuit of WO 2013/038176 is typically AC, power consumption is nevertheless limited by cost and size considerations. The present inventors have therefore recognised the desirability of obtaining a reference signal of as large amplitude as can be afforded by a limited power budget. The present inventors have also recognised that the current reference circuit should have a high output impedance to absorb voltage and impedance changes arising from changes in the load drawn current signal.
The present invention has been devised in the light of the above described problems. It is therefore an object for the present invention to provide current measurement apparatus which is configured to increase an amplitude of the reference signal in a power efficient fashion for accurate measurement of current, for example, in a circuit carrying mains current. It is another object for the present invention to provide a method of increasing an amplitude of the reference signal in a power efficient fashion to provide for accurate measurement of current, for example, in a circuit carrying mains current.